This is a docker container for the OpenWRT SDK.
$ docker run -it -v "$(pwd)/bin:/sek/bin/" aparcar/openwrt-sdk:18.06-ar71xx /bin/bash
Then you can build packages as follows:
# ./scripts/feeds update -a
# ./scripts/feeds install htop
# make package/htop/compile V=s
More information on how to use the SDK tool-chain can be found in the OpenWRT documentation.